1. Field of the Invention
The present invention relates to a voltage controlled oscillator, and particularly, to a voltage controlled oscillator using a thin film bulk acoustic resonator.
2. Description of the Background Art
Recently, information communication technology is greatly developed due to development of mobile communication terminal. Especially, as the mobile communication terminal becomes smaller, components used in the mobile communication terminal also become smaller and high-functional.
A voltage controlled oscillator among those components used in the mobile communication terminal is an essential component used for modulation/demodulation of high frequency signal, and requires wide frequency fine tuning range and low phase noise properties.
There are various methods for constructing the voltage controlled oscillator according to the conventional art, and the voltage controlled oscillator generally comprises an LC resonator including a variable capacitor and an inductor and an amplifier for oscillating a certain frequency. The voltage controlled oscillator according to the conventional art will be described as follows with reference to FIG. 1.
As shown therein, the voltage controlled oscillator comprises: an LC resonating circuit including a first variable capacitor C1 and a second variable capacitor C2 receiving control voltages (Vctrl) respectively and a first inductor L1 and a second inductor L2 connected to the first and second variable capacitors C1 and C2 in parallel to oscillate resonance frequency; an amplifier AMP1 for amplifying the resonance frequency generated by the LCD resonance circuit to a certain frequency through positive feedback; and an output buffer 1 for outputting the frequency amplified by the amplifier AMP1 after buffering it. Herein, the phase noise is decided according to the variable capacitors C1 and C2 and Q value of the inductors L1 and L2. Therefore, in order to minimize the phase noise, the variable capacitors C1 and C2 and the inductors L1 and L2 having high Q value should be used.
A varactor using a PIN diode or a capacitor fabricated in a micro machining technology is used as the variable capacitor C1 or C2, and a chip inductor, a wired inductor or an inductor fabricated in the micro machining technology is used as the inductor L1 or L2.
The component having the largest Q value among the capacitors and the inductors is the capacitor and the inductor fabricated in the micro machining technology, the Q value of the capacitor fabricated in the micro machining technology is about 200˜300, and the Q value of the inductor fabricated in the micro machining technology is about 100˜200.
However, the capacitor and the inductor fabricated in the micro machining technology can not be integrated on a same substrate. That is, in the conventional art, it is not easy to process when the capacitor fabricated in the micro machining technology is used, and accordingly, the chip inductor or the wired inductor is used. In case that the inductor fabricated in the micro machining technology is used, the LC resonator is fabricated using the varactor of the semiconductor substrate.
Therefore, in case that the capacitor fabricated in the micro machining technology is used and the chip inductor is used, an additional chip should be used. Also, in case that the capacitor fabricated in the micro machining technology and the wired inductor are used, reliability of the wired inductor is lowered and the mass production can not be performed. Also, a case that uses the inductor fabricated in the micro machining technology and the varactor having tens of Q value is actively being researched, however, the high frequency property of the inductor and of the varactor is limited.
Also, the method for fabricating the voltage controlled oscillator using the LC resonator is the most generated method, however, it is not easy to fabricate the oscillator and the property of the voltage controlled oscillator is degraded.
On the other hand, an oscillator using a crystal resonator and a surface acoustic wave (SAW) resonator instead of using the LC resonator is widely used as considering the above problems, however, it can not be applied to the voltage controlled oscillator which should control the oscillating frequency using the bias voltage. For example, the resonance frequency of the crystal resonator is decided by cutting direction and thickness of the crystal, and therefore, the resonance frequency can not be controlled by the bias voltage. The resonance frequency of the SAW resonator is decided by a shape of electrode formed on a surface of a piezoelectric material, and therefore, the resonance frequency can not be controlled by the bias voltage.
As described above, the voltage controlled oscillator according to the conventional art should use the capacitor and the inductor having larger Q values in order to minimize the phase noise, however, the capacitor and the inductor fabricated in the micro machining technology having the largest Q value can not be fabricated on same substrate, and therefore, the voltage controlled oscillator can not be realized easily. And in case that the capacitor and the inductor of different forms are used together, the property of the voltage controlled oscillator is degraded.